Liquid distributor for heat and mass exchange device, distillation column comprising such a distributor and use of such a column for distillation of air

ABSTRACT

The distributor includes a pattern of irrigation points with advantageously substantially constant surface pitch over most of the irrigation area, the pattern being, typically, regular in a main region of the distributor and irregular, with associated surface regions which are also irregular in a junction region, the irrigation points delivering predetermined flow rates defined by calibrated orifices which have cross-sections chosen from a predetermined group of n cross-sections, n not being greater than four and advantageously not greater than two. This distributor is particularly applicable to packed air distillation columns.

The present invention relates to a liquid distributor for a heat andmass exchange device, the device having an upper interface constitutingan irrigation area, the distributor comprising gas passages and aplurality of liquid passages.

Distributors for heat and mass exchange devices of this type aredescribed, for example, in documents FR-A-2,121,185 and EP-A-0,491,591.In practice, in particular in packed air distillation columns, use ismade of two distributors, as described in FIGS. 1 to 4 hereinbelow. Afirst type, described in FIGS. 1 and 2 includes, arranged transverselyin the shell ring 1 of the column, a plane structure 2, advantageouslyconstituting the coplanar bottoms of attached parallel chutes extendingabove the mass and heat exchange device 3, advantageously consisting ofpacking, in bulk or structured--typically with so-calledcorrugated/crossed structure, as described in document EP-A-0,367,817 inthe name of the Applicant Company. The structure 2, with the chuteswhich communicate with one another, especially at the periphery of thestructure 2, forms a reserve for accumulation of a quantity of liquid Land includes numerous orifices 4 for distributing the liquid undergravity onto the irrigation area of the device 3. The structure 2includes a series of circular or oblong shafts 5, as represented in FIG.1, allowing passage of gas upwards through the distributor. In the othertype of distributor, represented in FIGS. 3 and 4, the liquiddistributor comprises a series of chutes or of parallel tubes 20 whichare spaced apart from one another, centrally supplied with liquid andoptionally connected by one or more transverse chutes 21, the variouschutes forming the liquid reserve L and forming between them spaces 50for passage of the gas. The chutes 20 form, at their lower end, asurface 2' which is parallel to the upper surface of the device 3 and inwhich the discharge orifices 4 are formed. In one or other of the twotypes, the orifices 4 are formed, as represented in FIGS. 1 and 3, inregular patterns (orifices aligned in parallel rows) apart from thedetailed arrangement of the discharge regions in the vicinity of theshell ring 1. For reasons of ease of manufacture, the orifices 4preferably have identical cross-sections. Nevertheless, as representedin FIG. 5, in conjunction with the type of distributor according toFIGS. 1 and 2, if the orifices 4 are distributed in a regular patternwith a regular grid of irrigation surfaces, represented by the dottedrectangles 6 in FIG. 5, it is observed that, in the vicinity of theperipheral contour of the distributor, typically defined by the shellring 1 of a column, some regions of the grid of orifices 4, such asthose represented at 7, have an "amputated" surface, which indicatesthat the grid cells in question have a local flow rate comparativelygreater than that of the other orifices in the regular grid, whereasother regions, such as those represented at 8, have no dischargeorifices and therefore constitute "dry" regions, these two conditionscontributing to local degradation of the ratio of the gas flow rate tothe liquid flow rate in the column. One solution for overcoming thesedrawbacks must consist in increasing the number of orifices so as toreduce the pitch of the regular pattern of points, accordingly reducingthe diameter of the orifices in order to retain the liquid buffer L inthe distributor. This solution is, however, unsuitable insofar as itconsiderably increases the manufacturing costs and limits the precisionof the liquid discharge flow rate because of the difficulties ofproducing precise orifices with small cross-section, which, on thecontrary, militates in favour of the production of large-diameterorifices, since the problems of liquid flow rate are made increasinglymore acute by the enhanced risks of blocking or clogging of the orificeswith small cross-section. Another approach must consist in producing, inthe peripheral region, orifices with different adapted diameters:smaller in regions such as 7 or increased in the vicinity of regionssuch as 8. This solution is also ill-suited insofar as it would requirethe production of a large number, generally greater than 10, ofdifferent orifice cross-sections, which would considerably increase themanufacturing costs because of the requirement of providing multiple orcomplex drilling tools, moreover to the detriment of the precision ofthe liquid flow rate, and by exposing the orifices with smallercross-sections to the risk of blockage.

The object of the present invention is therefore to provide a precisiondistributor which makes it possible, in a simple, flexible, reliable andinexpensive manner, to increase the regularity of the liquid flow ratedischarged, while reducing the risks of blockage.

For this purpose, according to one characteristic of the invention,characterized in that the liquid passages are defined by calibratedorifices distributed in a pattern of irrigation points which is at leastpartially irregular, the orifices having a cross-section chosen from agroup of n predetermined different cross-sections, n not being greaterthan 4, and advantageously not being greater than 2, each orifice beingassociated with an individual surface having an area corresponding tothe cross-section of the associated orifice.

According to one particular characteristic, the orifices have asubstantially constant cross-section, corresponding to a nominal passagecross-section of a circular orifice with diameter typically lyingbetween 2 and 10 mm, advantageously between 3 and 5 mm, and aredistributed in a pattern of irrigation points with substantiallyconstant surface pitch over the irrigation area.

This arrangement according to the invention makes it possibleefficiently to deal with the problems of the edges of the distributor,with a reduced range of orifice cross-sections, especially with orificeswith single passage cross-section, while simultaneously making itpossible to optimize the number of irrigation points per unit irrigationsurface area, while retaining a relatively large nominal orifice passagecross-section and therefore decreasing their sensitivity to blockage.

According to other characteristics of the invention, the distributorincludes:

a plane structure in which the orifices are formed according to the saidpattern;

a series of interconnecting chutes in which the orifices are formedaccording to the said pattern.

In the context of the present invention, the term "chute" means profileswith angled cross-section and flat bottom, as represented, or withrounded cross-section, for example tubes, the lower parts of which aresubstantially tangential to one and the same horizontal plane.

A further subject of the present invention is a distillation column, inparticular an air dispersion column, equipped with such a distributorcombined with a heat and mass exchange device of the packed type,advantageously with structured packing.

Other characteristics and advantages of the present invention willemerge from the following description of embodiments which are given byway of illustration but without any limitation, in conjunction with theattached drawings, in which:

FIGS. 1 and 3 are plan views of two main embodiments of liquiddistributors for distillation columns;

FIGS. 2 and 4 are sectional views on the section planes II--II and IV-IVin FIGS. 1 and 3, respectively;

FIG. 5 is a diagrammatic plan view of a conventional distributor,showing the distribution of the liquid passages;

FIG. 6 is a similar view to FIG. 5, illustrating one distribution of theliquid passages according to one embodiment of the invention, applied toa distributor with plane structure;

FIG. 7 is a plan view similar to FIG. 6, showing the distribution of theliquid passages in a distributor with chutes according to one embodimentof the invention; and

FIG. 8 is a plan view of an alternative embodiment of a distributor withchutes according to the invention.

As shown in the embodiment of FIG. 6, in a distributor of type withplane structure according to the invention, there is advantageously, inthe central part of the structure 2, a distribution of orifices 4 in aregular pattern, here orthogonal, each being at the centre of gravity ofan individual regular surface region 6, here quadrilateral. According toone aspect of the invention, the central pattern of orifices 4 with agrid of regular surface regions 6 is supplemented, at its periphery, bya disordered pattern of orifices 4' arranged at the centre of gravity ofsurface regions 6' having, in the embodiment represented, mostlysubstantially the same area as the regular regions 6, but with irregularshapes, the orifices 4' here mostly having the same cross-section as theorifices 4 of the regular pattern. On the left of FIG. 6, however, twoorifices 4" have been represented with the same cross-section which isless than that of the orifices 4 and 4' with which the surface regions6" are associated, which regions have a reduced surface areaproportional to the reduced cross-sectional area of the orifices 4". Fora given total irrigation area, depending on the diameter of the column,and for a predetermined orifice 4, 4' nominal diameter which typicallylies between 3 and 5 mm, it is possible to define a total number ofidentical orifices 4, 4' and therefore a corresponding individualsurface area, the shapes of the individual peripheral surfaces 6' and 6"being determined by step-by-step calculation of surface areas over theresidual irrigation area around the central area having regular surfaces6. According to the invention, other orifices (not shown) havingcross-sections greater than those of the orifices 4, 4' or less thanthose of the orifices 4" may be provided locally, at the centre ofgravity of individual surfaces with areas corresponding to thecross-sections of these orifices. The number of different orificecross-sections is not greater than 4, and in practice, is not greaterthan 3. The ratio of the largest orifice cross-section to the smallestorifice cross-section (in practice corresponding to a diameter of 1 mm)does not exceed 100, and typically does not exceed 50.

As shown by FIG. 7, in one embodiment having separate chutes 20, thelatter are, according to one aspect of the invention, dimensioned andshaped so that their free ends 200 substantially match the profile ofthe column shell ring 1, in immediate proximity thereto, and theindividual surfaces 6 are determined as a function of the distributionof the orifices 4 having the same nominal cross-section, which areformed in the bottom of the chutes 20, this time starting from the ends200, that is to say the side edges of the distributor. As shown by FIG.7, the distribution of the orifices 4 is regular (here in circular arcsand in alignment) over the side parts of the distributor, thecorresponding surface regions being mostly regular, and thisdistribution reproduces the problem of adjusting the individual surfaceareas in the region of the distribution or equilibration chute 21 where,as in the case of the peripheral region in FIG. 6, the individualsurfaces 6' of the orifices 4' with irregular pitch have variablecontours which are determined by step-by-step calculation of surfaceareas and conserve, in the case of orifices 4 and 4' having the samenominal cross-section, the same surface area as the regular surfaces 6.Here again, orifices 4" having reduced cross-section with respect tothat of the orifices 4, 4', and associated with surface regions 6" whichalso have reduced surface area, have been represented.

Depending on the geometry of the distributor, there may be cause tooffset the irrigation points with respect to the chutes 20 in which thecalibrated orifices 4 are made. In the embodiment of FIG. 8, dischargein the vicinity of the shell ring 1 is thus ensured, at least in part,by the outlets 41, for example of the overspill type, which extend belowthe chutes 20 and outside the latter, in order to make irrigation points4' outside the chutes, but ones which nevertheless ensure a dischargeflow rate advantageously identical to that of the orifices 4 of thechutes, each being supplied by an orifice 40 made in one chute andhaving the same cross-section as the orifices 4, the distribution of theirrigation points 4' being determined in the same manner as for those inthe case of FIGS. 6 and 7.

Although the invention has been described with reference to particularembodiments, it is in no way limited thereby but, on the contrary, issusceptible of amendments and variants which may occur to the personskilled in the art.

I claim:
 1. A liquid distributor for a heat and mass exchange device having an upper interface forming an irrigation area, the liquid distributor comprising gas passages and a plurality of liquid passages, said liquid passages being defined by calibrated orifices distributed in a pattern of irrigation points, which pattern is at least partially irregular, the orifices having a cross-section chosen from a group of n predetermined different cross-sections, n being an integer not greater than 4, each orifice being associated with an individual surface having an area corresponding to the cross-section of its associated orifice.
 2. The distributor according to claim 1, wherein the ratio between the maximum cross-section and the minimum cross-section in the group of n cross-sections is less than
 100. 3. The distributor according to claim 1 wherein n is not greater than
 2. 4. The distributor according to claim 1, further including a plane structure in which the orifices are formed in said pattern.
 5. The distributor according to claim 1, further including a regular pattern of irrigation points disposed in a central part of said pattern.
 6. The distributor according to claim 1, further including a series of parallel, interconnecting chutes in which the orifices are formed in said pattern.
 7. The distributor according to claim 6 further including at least one discharge passage extending between an orifice formed in one of said chutes and an irrigation point offset from said one chute.
 8. The distributor according to claim 6, wherein at least some of the chutes have one end extending into the immediate vicinity of a peripheral shell ring which encloses the distributor and the exchange device.
 9. The distributor according to claim 8, further including a regular pattern of irrigation points located in lateral regions close to said one end of the chutes.
 10. A distillation column associated with a heat and mass exchange device of the packed type, said distillation column having a liquid distributor including an upper interface comprised of an irrigation area containing gas passages and a plurality of liquid passages, said liquid passages being defined by calibrated orifices distributed in a pattern of irrigation points, which pattern is at least partially irregular, the orifices having a cross-section chosen from a group of n predetermined different cross-sections, n being an integer greater than 1 and not greater than 4, each orifice being associated with an individual surface having an area corresponding to the cross-section of its associated orifice.
 11. The column according to claim 10, wherein the heat and mass exchange device is of the structured packing type. 